This invention relates in general to cryogenic cooling systems. More specifically it relates to miniature split cryogenic systems with the compressor section separated from the cooling section and the compressor operating with a split phase.
Miniature cryogenic cooling systems are widely use to cool crystals used as transducers. The cooling reduces lattice vibrations which would otherwise obscure or degrade the quality of the output signal. A particularly important application is in the cooling of the infrared sensing material for night-vision or heat-seeking devices. Miniature cryogenic systems are also useful for medical applications such as the freezing of small quantities of brain tissue in the treatment of Parkinson's disease. While the requirements of a cryogenic system will vary depending on the use, some typical considerations are its operating efficiency, durability, compactness, weight, microphonics (typically vibrations from the compressor motor, a compressed gas, or the physical impact of moving components in the system), and thermophonics (which may be defined as electronic white noise generated by heat conducted from the warmer to the colder portion of the system). For applications involving infrared sensors for airborne devices, all of these factors are important.
One system presently used in military equipment and dicussed in an article by Franz Chellis, "Comparing Closed-Cycle Cryocoolers" in the November, 1979 issue of Electro-Optical Systems Design is an integral Stirling cycle system, that is, one with the compressor and expander forming a single mechanical package. The expander is an elongated cylindrical structure commonly termed a "cold finger" since it is finger shaped and the cryogenic cooling occurs at its extreme tip end. A motor drives a compressor piston and the displacer through a lubricated helical gear. A flywheel is mounted on the motor shaft in an attempt to control the vibrations produced by the motor and to prevent their transmission to the expander. The piston compresses a working fluid, typically a low freezing temperature gas such as helium, that is conducted to the expander. The pressurized gas moves through an axially reciprocating displacer in a manner that cools a working volume between an end of the displacer and an end plate of the cold finger housing. The gas flow to this region is through a regenerative material, or regenerator, held in the displacer. The regenerator acts as a heat exchanger and maintains a temperature gradient between the cold tip of the cold finger and the gas inlet. The regenerator is often a metallic screen or small spheres of copper or nickel. In any event, the regenerator has a comparatively large mass.
A major disadvantage of this device is that despite the flywheel, there is a significant transmission of motor vibration to the cold finger. For many applications, the irreducible level of vibration is unacceptable. Another problem inherent in this integral system is that the lubricant for the helical gear breaks down and can recondense to clog fluid flow passages. Seal wear has the same disadvantage and in addition it can allow fluid leakage that detracts from the efficiency of the system. This system is also comparatively heavy (approximately four pounds), bulky (the compressor section measures approximately 41/2 inches by 31/2 inches and the cold finger extends three to four inches), and has a typical operating life of only 300 to 500 hours. The weight, bulkiness and microphonics problems of this system make it particularly poor for certain uses in airborne missiles.
U.S Pat. No. 4,078,389 to Bamberg describes another cooling system which in different embodiments uses either the Stirling cycle or the Vuillieumier cycle. Bamberg attempts to solve problems associated with connecting a rotating drive shaft to a pair of linearly reciprocatng pistons 180.degree. OFF-phase with each other. In the Stirling cycle form, an eccentrically mounted crank arm drives a doubly articulated connecting link which in turn drives the pistons and the displacer in a generally linear path. In a Vuillieumier cycle form, a scotch yoke connected to a rotating, eccentrically mounted crank to produces a generally linear drive force for a pair of pistons. These arrangements have two major disadvantages. First, as in the above described apparatus, the motor is mechanically coupled directly to the displacer. Control of microphonics is therefore extremely difficult. Second, the drive systems apply the drive force over a comparatively long moment arm which develops a significant side thrust on the main seals. As a result, they are prone to rapid wear and failure.
In an attempt to isolate the vibration of the compressor from the cold finger, split Stirling devices are known which separate the compressor section from the cold finger by conduits that carry the working fluid. U.S. Pat. Nos. 4,090,859 and 3,991,586 describe a single compressor, single split Stirling system. U.S. Pat. Nos. 4,206,609 and 4,092,833 describe compound-compressor dual-split Stirling systems. A common design problem of these systems is the control of the acoustic noise generated by a free oscillating dispenser slamming back and forth against containment surfaces within the cold finger. Noise is especially troublesome in any single split Stirling system.
To control the movement of the displacer, U.S. Pat. No. 4,090,859 uses an enclosed pneumatic air spring located at the end of the cold finger opposite the cryogenically cooled end. U.S. Pat. No. 3,991,586 uses a spring and a solenoid to control the movement of the displacer. Both systems are, nevertheless, plagued by the host of problems recited in the '609 patent. For example, the high frequency of operation of the device leads to large acceleration and deceleration forces. The large forces associated with oscillating the displacer produce vibration microphonics despite the effects of pneumatic and mechanical springs. Another problem is the wear of friction seals particularly where the degree of friction is important in controlling the displacer motion. Still another problem is heat due to friction or due to gas compression in the pneumatic volume. Radiating fins can be used to assist dissipation, but they increase the size of the device. Also, no known single-split Stirling system has an acceptable repeatable operating life. A typical operating life is 10 hours.
U.S. Pat. Nos. 4,092,833 and 4,206,609, both to Durenec, describe systems where not only is the compressor physically separated from the cold finger, but also it operates on a split phase. The compressor has two pistons that develop gas pressure in separate conduits that are connected to separate chambers in the cold finger. In the '833 patent the compressor cylinders operate 180.degree. out of phase and one piston has a smaller effective area than the other piston, This results in a dual-split, "push-pull" mode of operation at the cold finger one compressor cylinder is developing a suction in one chamber that assists the compression developed by the other compressor cylinder in the other chamber. In the '609 patent, two pistons, again of different size, operate 90.degree. out of phase with the larger piston cylinder driving waves of compressed gas through a large displacer to the volume to be cooled. The smaller piston feeds a smaller, stationary displacer received in the end of the large displacer opposite the cooled end. This arrangement also provides a "push-pull" mode of operation. In one form, the conduit from the large piston cylinder to the main displacer is wrapped around the "warm" end of main displacer where a small displacer is enclosed, to precool the gas supplied to the main displacer. The '609 patent also describes a multi-stage displacer where the working gas flows toward the cooled end through multiple regenerators of decreasing volume. The regenerator in the displacers in all of the single-split and dual-split systems discussed above is carried within the displacer which oscillates primarily in response to applied fluid pressures.
The Durenec designs, however, also have drawbacks. A principal problem with the '833 arrangement is that the pressure waves developed in the "rear" volume do not effectively control microphonics and overcome acceleration and deceleration problems. The principal drawback of the '609 system is that the system does not effectively control the vibration problems associated with a displacer having a large mass and oscillating at a high frequency.
It is therefore a principal object of this invention to provide a miniature cryogenic system which is highly efficient, compact and characterized by a low level of microphonics and thermophonics.
Another principal object is to provide such a system operating on a dual-split, "compound" Stirling cycle that has a comparatively long operating life.
A further object is to provide such a system, including a split-phase compressor, which is highly compact and has a significant weight reduction as compared to known systems.
Another object is to provide a system with the foregoing advantages that reduces seal wear and avoids problems associated with lubricant breakdown.
A still further object is to provide a system with all of these advantages that is formed of conventional materials and has a competitive cost of manufacture.